As well known, the transmission path used for transmitting signals in telecommunication connections causes interference to telecommunications. This occurs irrespective of the physical form of the transmission path, i.e. whether the transmission path is a radio connection, an optical fibre or a copper cable, for instance. Particularly in radio communications, situations often occur where the quality of the transmission path varies from one connection to another and also during a connection.
Fading on a radio path is a typical phenomenon causing changes in a transmission channel. Other simultaneous connections may also cause interference, which may change as a function of time and place.
In a typical radio communication environment, signals between a transmitter and a receiver propagate along several paths. This multipath propagation is mainly caused by the signal being reflected from surrounding surfaces. Signals that have propagated along different paths arrive at the receiver at different times due to different propagation delays. Different methods have been developed to compensate for the fading caused by the multipath propagation.
A solution to the problem is to use diversity in the transmitter. In time diversity, interleaving and encoding are used to achieve time-based diversity in the signal to be transmitted. However, a drawback is delays in transmission, especially when the channel is a slowly fading one. In frequency diversity, in turn, the signal is transmitted simultaneously at several frequencies. However, this is an inefficient method when the channel has a wide coherence bandwidth.
In antenna diversity, a signal is transmitted and/or received by means of more than one antenna. Hereby the signal components that have multipath-propagated via different channels will probably not be interfered with by a simultaneous fade. In reception diversity, the transmitted signal is received by two or more antennas with different locations. A drawback of reception diversity is the difficulty of implementing two antennas in a terminal equipment that should be as small as possible. Transmit diversity includes transmitting the same signal to the receiver by means of two or more different antennas. The receiver combines the signals for example by means of the MLSE (Maximum Likelihood Sequence Estimator) or the MMSE (Minimum Mean Square Error) methods. In downlink communication in mobile telephone systems, transmit diversity is more applicable than reception diversity, since it is easier to set up several antennas in a base station than in a terminal equipment.
The cellular radio systems that are currently under development, such as UMTS, also provide the possibility of using two transmit antennas. An example of such systems is the UMTS standard version Release 99. However, the use of even more antennas, such as four antennas, for transmit diversity is also being developed. Such methods will possibly be provided in subsequent UMTS standard versions. In transmit diversity according to the Release 99 version, each channel is transmitted with two radiation patterns.
A particular feature of transmit diversity is the required transmission of pilot signals, i.e. typically one additional pilot for each additional diversity. Pilot signals are particularly used in base station transmission in CDMA systems. There are various types of pilot signals. Firstly, there are common pilots, which are intended for all the terminal equipments collectively. Secondly, there are dedicated pilots, which are included in the transmission of one user signal. Common pilot signals are used in the terminal equipment for forming a channel estimate for a dedicated channel. Similarly, several other measurements are also carried out on a common pilot signal, such as handover, synchronization and idle mode cell selection measurements. If a terminal equipment does not have a dedicated channel, the channel estimate is formed entirely based on the common pilot There can be further two types of common pilot signals, i.e. primary and secondary common pilots. A primary common pilot is transmitted with an omnidirectional radiation pattern in the cell, such that the pilot can be received by all the terminal equipments in the cell. A primary common pilot is typically transmitted at a power level that constitutes a significant proportion (e.g. 10%) of the entire base station transmit power. A primary common pilot is transmitted by means of a particular channelization code and a scrambling code. The use of a primary common pilot is significant in measurements related to handover and cell selection. There can be several secondary common pilots in the cell area, and they are transmitted e.g. with beam-shaped radiation patterns to areas with congestion, for instance. A secondary common pilot is transmitted by means of a channelization code of a particular length (256), but the code itself can be selected freely.
In the case of one transmit antenna one primary common pilot is transmitted. For example in the UMTS, the pilot symbol sequence is then of the form AAAA, where A=1+j.
In the case of two-antenna transmit diversity, a primary common pilot is transmitted at the same power level from each antenna. The pilot symbol sequence in one antenna is thus of the form AAAA and in the other antenna A-A-AA. The channelization code and the scrambling code are the same.
It is possible to introduce into the market such terminal equipments that are capable of receiving base station transmissions utilizing transmit diversity of at most two antennas. Such equipments include terminal equipments according to the UMTS Release 99 version. When the diversity technique is further developed in order to utilize more than two antennas, it should be ensured that terminal equipments capable of receiving transmissions transmitted via only two transmission routes receive high-quality service, whereas terminal equipments that are capable of receiving a signal transmitted over more than two transmission routes will be able to utilize transmit diversity to the maximum. Particularly transmission of pilot signals is problematic since primary pilot signals can only be transmitted via two transmission routes.
A prior art arrangement for transmitting pilot signals in connection with four antennas is to transmit a primary common pilot signal via two antennas and a secondary common pilot via two other antennas, all the transmissions taking place at the same power level:
Transmit antennaPilot sequenceSignal1AAPrimary2A-A/-AAPrimary3AASecondary4A-A/-AASecondary
A drawback of this method is that the terminal equipments capable of receiving a signal transmitted with only two antennas lose some of the pilot signal power. This creates problems for the terminal equipments since the measurements to be carried out on the pilot (for handover and cell selection) will become more difficult as the power of the pilot signal decreases. Furthermore, this method cannot be applied to more than four transmit antennas without adding new channelization codes.